An internal combustion engine generally operates in four modes; an intake mode, a compression mode, a combustion mode and an exhaust mode. During reverse rotation of an engine, the engine cycle executes in a reverse order whereby the compression mode is followed by the intake mode. For example, when an engine that is stopped begins to start again, the engine may have a cylinder that was in a compression mode at the moment of stopping. Compression pressure in the cylinder may push a piston in reverse toward bottom dead center (BDC). When engine speed increases, a cylinder with injected fuel may experience ignition and the reverse rotation may be accelerated.
It is unlikely that a conventional engine will rotate in reverse for a long period of time. Torque control systems are capable of limiting the duration of the reverse rotation. However, the problem arises more frequently in hybrid electric propulsion systems. An external force (such as an electric motor) can rotate the internal combustion engine in reverse for longer durations at higher speeds. Conventional torque control systems are not able to control torque under these conditions.
If reverse rotation occurs, engine components such as the intake manifold can be damaged. Reverse rotation may cause a compressed air/fuel mixture to flow back into the intake manifold during the intake stroke through an open intake valve. Pressure in the intake manifold increases. If further reverse rotation occurs, pressure may increase further and cause damage to the intake manifold.